1. Field of the Invention
The present invention relates to a constant-speed running controller which controls a throttle valve opening degree to maintain the running speed of a vehicle on a constant level.
2. Description of the Prior Art
Japanese Unexamined Patent Publication (Kokai) No. 76429/1985 discloses a constant-speed running controller which controls a throttle valve opening degree according to the condition of the road to maintain the present running speed of a vehicle for constant-speed running.
This known constant-speed running controller comprises a running speed detector, a storage device for storing a predetermined target running speed, an actuator for operating the throttle valve, a negative pressure applying device for applying a negative pressure to the actuator, including a negative pressure source and a vacuum pump, an electronic control unit which compares the present running speed of the vehicle with the predetermined target running speed stored in the storage device and controls the actuator so that the difference between the predetermined running speed and the present running speed is reduced to zero, and a pump driving device which starts driving the vacuum pump upon the increase of the controlled deviation, namely, the deviation of the present running speed from the predetermined target running speed, beyond a predetermined limit of deviation.
This known constant-speed running controller, however, is unable to control the constant-speed running mode of the vehicle efficiently at a high accuracy according to the running condition of the vehicle, because actual controlled running speed of the vehicle deviates from a predetermined target running speed due to the imperfect operating characteristics and unsatisfactory accuracy of the component control elements such as the running speed detector, the throttle valve and the actuator.